A. Although respiratory sinus arrhythmias (RSA) are known to decrease with advanc-ing age, the independent effects of age, gender, conditioning status and body com-position are unknown. The impact of age, fitness, gender and relative weight on resting heart rate variability was examined in 117 healthy normotensive adults ages 19-82 from the BLSA; heart rate variability was indexed by RSA, extracted from a 3-minute seated ECG using time domain digital filtering. By linear regression analy-sis, RSA varied inversely with age (r = -0.61, p <.001) and body mass index (r = -0.31, p <.01), directly with VO2max (r = 0.40, p <.001) and was unrelated to gender. Multiple regression analysis demonstrated that age and body mass but not VO2max were independent predictors of RSA. B. Whether arrhythmias or ST segment depression detected on 24-hr ambulatory ECG have prognostic significance in asymptomatic older adults, as they do in patients with known coronary artery disease, is not known. The prognostic significance of 24-hr ambulatory ECG recordings was determined in 98 healthy BLSA volunteers > 60 yr old. Over a 10 yr mean followup, coronary events (CE), defined as angina pectoris, myocardial infarction or cardiac death, developed in 14 individuals. The incidence of CE did not differ between subjects who developed frequent or complex supraventricular or ventricular arrhythmias and those who did not. However, CE developed in 6 of 16 subjects with horizontal or downsloping ST segment depression versus only 8 of 82 without ST depression, p <.05. Thus, silent ischemia, although infrequent in apparently healthy older subjects, may be the best predictor of subsequent CE. C. Almost no information is available regarding the prevalence, characteristics or prognostic significance of exercise-induced supraventricular arrhythmias. The prevalence, characteristics and prognosis of exercise-induced supraventricular tachycardia (SVT) was determined in 888 male and 555 female BLSA volunteers 17-94 yr old. SVT occurred in 85 subjects (5.9%) and was paroxysmal atrial tachycardia in 98% of SVT episodes. SVT cases were older than those without SVT (66.0q13.5 vs 49.7q18.1 yr, p <.001) but experienced a similar incidence of subsequent cardiac events as age-matched controls without SVT over a 5.3 yr mean followup period. De novo atrial fibrillation, however, developed in 10% of SVT cases versus only 2% of controls over followup (p < 0.10).